X-ray radiation emitted from an x-ray source may be filtered to modify the spectral output of the x-ray source. A first filter may be used to filter x-rays having a peak energy within a first range and a second filter may be used to filter x-rays having a peak energy within a second range. These filters are selected in a predetermined manner such that the first and second filters are always used to filter x-rays having the first and second peak energies, respectively.